The present invention relates to a beam hardening (BH) method for correcting (calibrating) the intensity of X-rays transmitted by a subject on the basis of phantom data, and a computed tomography (CT) system adopting the beam hardening method.
An X-ray CT system will be taken as an example of a CT system.
An X-ray source employed in an X-ray CT system generates X-rays that fall within a certain energy range. An absorption coefficient for X-rays to be transmitted by a subject depends on the energy in the X-rays. The larger a length in the subject over which X-rays are transmitted, the higher the average energy in transmitted X-rays. This phenomenon is referred to as a beam-hardening (BH) effect. Consequently, a proportional relationship is not established between the intensity of transmitted X-rays, that is, a projection information value produced from a signal detected by an X-ray detector included in the X-ray CT system, and the length in the subject over which X-rays are transmitted, but a linear relationship is.
The beam-hardening effect causes the cupping effect signifying that the intensity of the center of a reconstructed image produced by the X-ray CT system gets lower. A signal detected by an X-ray detector must therefore be corrected. A correction coefficient to be used to correct projection information values based on which a reconstructed image is produced to exhibit a uniform intensity is calculated in relation to each of the channels of the X-ray detector, whereby the correction is achieved.
For higher-precision correction, phantoms are used. Such phantoms include multiple cylindrical phantoms having circular sections and different diameters that are large enough to generally cover the entire field of view (FOV) (scan field) defined in the center of an X-ray field. Projection information acquired from the phantoms is used to precisely correct a correction coefficient (refer to, for example, Patent Document 1).
[Patent Document 1]
Japanese Unexamined Patent Publication No. Hei 7(1995)-171145
According to the foregoing method, when the projection information is acquired, the plurality of phantoms that has circular sections and different diameters must be disposed. Since the phantoms are large in size, disposing them is labor-intensive. Since scan must be repeated, much time is required. Besides, the projection information values cannot be corrected highly precisely in consideration of a non-linear effect attributable to the aforesaid beam-hardening effect.
Furthermore, in order to correct projection information values highly precisely, many different projection information values are needed in relation to each of the channels of an X-ray detector. Therefore, lots of phantoms that have circular sections and different diameters must be disposed in the center of a scan field between an X-ray tube and the X-ray detector, and then scanned.
In particular, in order to acquire calibration information for the X-ray CT system, two or three phantoms that have circular sections and diameters ranging from 20 cm to 50 cm are used and scanned for 100 min or more. The scan that persists for 100 min or more must be performed exclusively for precise correction. Thus, the calibration requires too much time and labor.